Nickel-cadmium batteries generally consist of a wound anode interleaved with a wound cathode, the wound anode and cathode being spaced apart at regular intervals in an electrolyte. The interval between the anode and cathode may be as small as 0.05 mm. Although it is desirable to place the cathode and anode close together to increase the load capacity of the battery, the electrodes must not touch to avoid producing a short circuit. To accomplish this end, separators made of suitable material are arranged between the anode and cathode to keep them apart. The separator material must be inert to the electrolyte and to the reactions occurring at the surfaces of the electrodes.
In addition, the separator material should be sufficiently elastic to conform to the shape of the electrode surfaces. Also the separator material should be sufficiently porous to allow unimpeded migration of ions between the electrodes, yet be able to filter out solid particles which separate from the electrodes and attempt to pass through the separator. The separator material further must be wettable by the liquid electrolyte to prevent the establishment of dry areas on the separator fabric. Finally, the separator should have the capacity to adsorb and store the liquid electrolyte.
Separator material made from woven fabric is disadvantageous because fabric stores insufficient quantities of the liquid electrolyte. Furthermore, because pores formed between the warp and weft of the fabric are large, solid particles which dislodge from the electrodes can pass through the fabric. Such particles accrete until a bridge is formed between an anode and cathode, giving rise to a short circuit in the battery.
It is known in the prior art that the foregoing disadvantages can be overcome by providing a battery separator material made from nonwoven nylon fabric. U.S. Pat. No. 3,344,013 to Fahrbach discloses a separator material for batteries comprising a highly porous and highly elastic structurally modified nonwoven fibrous material consisting of either nylon 6 (i.e., polycaprolactum) fibers or nylon 6--6 (i.e., polyamide) fibers or both. The separator material is manufactured by impregnating the fibrous material with a solvent consisting of a low-percentage aqueous salt solution to effect preliminary dissolution of the surface portions of nylon fibers. The impregnated nonwoven material is then squeezed under light pressure to remove excess salt solution therefrom and to initially strengthen the nonwoven material by fusing the fibers to each other at their superficially dissolved surface portions. Then the nonwoven material is dried and finally strengthened by heating.
U.S. Pat. No. 5,202,178 to Turner discloses a laminated nylon battery separator material for use in nickel-cadmium batteries. The laminate comprises a nonwoven web of nylon staple fibers sandwiched between a pair of webs of spunbonded nylon fibers. The staple web comprises nylon 6 and nylon 6,6 fibers. The spunbonded fibers are nylon 6,6. The three webs are laminated by passing them through a stack of heated calendar rolls. The maximum temperature of the stack of calendar rolls is greater than the softening temperature of the nylon 6 fibers, but less than the melting temperature of the nylon 6,6 fibers. Upon cooling, the webs of spunbonded fibers will be bonded to the staple web by the re-solidified nylon 6 fibers, whereby the laminated battery separator material is formed. In accordance with the preferred embodiment of Turner, the amount of nylon 6 may be in the range of 5-60 wt. % with the remainder being nylon 6,6 fibers.
U.S. Pat. No. 3,615,865 to Wetherell discloses a battery separator comprising a nonwoven mat of polypropylene fibers bonded with polyacrylic acid. In lieu of polypropylene fibers, polyethylene or polyamide fibers may be used.
U.S. Pat. No. 4,205,122 to Miura et al. discloses a method for manufacturing a battery separator material by subjecting an aqueous dispersion of olefinic resin fibers to a sheet-forming operation; drying the resulting wet nonwoven mat; and heat-treating the dried mat to form a self-supporting nonwoven mat. The drying and heat treatment of the nonwoven mat can be performed by passing it through a hot air dryer or "by means of dryers used in conventional papermaking machines, such as a Yankee dryer". After heat treatment, the mat is preferably calendared to increase the surface smoothness.
U.S. Pat. No. 4,216,280 to Kono et al. discloses a battery separator comprising glass fibers entangled to form a sheet and without use of a binder. Glass fibers of two types are dispersed in water and then sheet-formed by an ordinary papermaking method.
U.S. Pat. No. 4,216,281 to O'Rell et al. discloses a battery separator comprising 30-70% polyolefin synthetic pulp, 15-65% siliceous filler and 1-35% by weight of long fibers made of polyester or glass. Cellulose may be included in an amount up to 10%. The battery separator material is formed using standard papermaking equipment. The papermaking equipment disclosed in the O'Rell '281 patent comprises a pulper, a chest, a head box and a rotoformer drum which rotates in the head box to pick up slurry and form a web. The web is removed from the rotoformer drum and passed over a felt. The web is pressed by calendars. The calendared web is fed to an oven and then onto a series of heated cans. The cans feed to a windup station. In Example 1, the steam cans were operated at surface temperatures of about 270.degree. F. The steam cans both dried the web and increased fiber bonding.
U.S. Pat. No. 4,233,379 to Gross et al. discloses a battery separator comprising 30-80 wt. % perlite granules and 20-70 wt. % glass fibers. The compositions are formed into sheets of paper using conventional papermaking techniques, i.e., the glass fibers and perlite are dispersed in a water slurry and mixed; then the mixture is deposited from the water slurry onto a conventional papermaking screen or wire, as in a Fourdrinier machine or a Rotoformer machine, to form a matted paper.
U.S. Pat. No. 4,279,979 to Benson et al. discloses a battery separator material. The major fibrous component of the material is polyolefin pulp having a prefused microfibrillar structure similar to wood pulp. The minor fibrous component is a high-tenacity polyamide fiber having a length greater than about 6 mm. The material is heat bonded by partial fusion of the microfibrillar polyolefin. The preferred polyamide is nylon, the amount of nylon fibers being preferably in the range of 10-25%, although the Benson patent states that 10-50% can be employed with satisfactory results. Alternatively, polyolefin staple fibers can be added with the polyamide fibers. The sheet material is made in accordance with conventional papermaking techniques. The major and minor fiber components are interentangled to provide sufficient structural integrity without the use of binders. The fibrous web thus formed is typically dried at drying temperatures of about 220.degree. F. and higher. In this way the polyolefin pulp softens during drying and partially exceeds its fusion temperature, thereby bonding the web. Then the thickness of the sheet material is reduced by calendaring, which also has the effect of imparting added strength to the sheet material.
U.S. Pat. No. 4,699,858 to Nakao et al. discloses a battery separator formed of a nonwoven fabric of polyamide fibers wherein the polyamide has a CONH/CH.sub.2 ratio of from 1/9 to 1/12.
U.S. Pat. No. 5,091,275 to Brecht et al. discloses a battery separator material made of a mat of glass microfibers and a binder. The glass mat is formed on a conventional papermaking machine, such as a Fourdrinier machine. The mat is then moved through an impregnating bath of an aqueous mixture of a binder.
U.S. Pat. No. 5,141,523 to Catotti et al. discloses an electrochemical cell having separator layers formed of nonwoven mats of 67% nylon 6,6 and 33% nylon 6.
U.S. Pat. No. 5,158,844 to Hagens et al. discloses a battery separator in the form of a nonwoven fibrous web of water-dispersible fibers incorporating up to 65 wt. % of fibers having a high cross-sectional aspect ratio. The high aspect ratio fibers include collapsible hollow fibers and ribbon fibers that have a width 5 to 10 times greater than their thickness. The separator is produced using conventional papermaking techniques. The fibers are preferably a mixture of polyvinyl alcohol and rayon.
U.S. Pat. No. 5,281,498 to Muto et al. discloses a sheet-like battery separator for a lead acid battery. The sheet material is made from glass fibers on a papermaking machine using a wet method.
U.S. Pat. No. 5,436,094 to Horimoto et al. discloses a bulky synthetic pulp sheet useful as a separator for sealed lead batteries. The pulp sheet contains 5-95 wt. % of a synthetic pulp and 5-50 wt. % of a polymer binder. The sheet is made by subjecting a mixture of synthetic pulp and fibrous binder to wet-laid sheet-making followed by a heat treatment. The pulp can consist of polyethylene, polypropylene, polyester, nylon or other polymers. The binder may take the form of synthetic pulps, synthetic fibers, sheath-core type composite fibers, resin powders and emulsions. The type of binder selected is dependent on which kind of synthetic pulp is used as the chief material.